1. Field of the Invention
This invention relates to a hologram recording and reconstructing method, wherein a Bi.sub.12 SiO.sub.20 single crystal or the like are used as a material for recording of a volume hologram, an apparatus therefor, an apparatus for projecting reconstruction rays, and a hologram element and a manufacturing method thereof.
2. Description of the Related Art
The holography has been applied, as a technique for reconstructing a perfect wavefront, and in measurement of interference, optical information processing and optical elements. In order to conduct a hologram recording or holographic interference method, nowadays, silver salt photosensitive materials have generally been used, which are excellent in sensitivity and resolving power. However, when the silver salt photosensitive materials are used, naturally a process of development treatment is required. Therefore, photorefractive crystals (crystals showing a photoinducing refractive effect) beginning with Bi.sub.12 SiO.sub.20 single crystals have been extensively investigated as a real-time hologram (RH) element (see, e.g. Study Report on Laser Science, Mar. pp. 1-9, (1990), "Holographic Recording Property of BSO Single Crystal"). These materials, since they can record a hologram and reconstruct as it stands only by impressing voltage, have been attracting attention particularly as a material for real-time hologram elements. In the above literature, object rays and reference rays are projected on a (110)-orientated crystal face of the Bi.sub.12 SiO.sub.20 single crystal, an electrode is attached to both terminal surfaces of the (110) face and an electric field of several kV/cm is impressed in the direction perpendicular to interference fringes.
On the other hand, as to the dimension of the hologram, though it is restricted according to the use object in various uses, such as measurement of interference, optical information processing and optical elements, large-size elements have not necessarily been required. However, in the use for three-dimensional display that has been long-expected to be put to practical use, it is absolutely necessary to increase the size of the hologram to at least a certain extent. Because, in the use for the three-dimensional display, it is necessary to utilize binocular parallax for observers' stereoscopical recognition and, therefore, the size of the hologram must be larger than the distance of both eyes (about 50 mm).
However, the upper limit of the size of real-time hologram elements, since it is restricted physically by the shape and size of the BSO single crystal that is a recording member for recording interference fringes, has been substantially at most ten and several mm.times.ten and several mm. Due to this restriction of the dimension, the use of the real-time hologram elements has so far been limited only to the measurement of interference and the optical information processing. However, the real-time hologram is a technique which, in particular, has been eagerly desired to be put to practical use in future, and its application as an output apparatus of a three-dimensional image display system, substantially, such as a three-dimensional CAD system or the like, has been waited for. Therefore, large-size real-time hologram elements have been strongly expected.
The Bi.sub.12 SiO.sub.20 single crystals, since they have a low heat conductivity as compared with general oxide single crystals, are difficult to give off latent heat evolving following crystallization. Therefore, during growing of the crystals, strains or cracks associated with thermal distribution within the crystals are readily formed and therefore the large-sizing has been deemed to be difficult. However, the researchers belonging to this applicants' company, disclosed a process suited for pulling up single crystals by means of a pull-up method (Czochralski technique) (see, Journal of Japan Crystal Growth Society, Vol. 17, No. 2 (1990), pp. 60-66; Japanese Patent Kokai Nos. 64-18993 and 1-234399).